Problem: $ B = \left[\begin{array}{rr}1 & 3 \\ 0 & -2 \\ 5 & 0\end{array}\right]$ $ w = \left[\begin{array}{r}-1 \\ -2\end{array}\right]$ What is $ B w$ ?
Solution: Because $ B$ has dimensions $(3\times2)$ and $ w$ has dimensions $(2\times1)$ , the answer matrix will have dimensions $(3\times1)$ $ B w = \left[\begin{array}{rr}{1} & {3} \\ {0} & {-2} \\ \color{gray}{5} & \color{gray}{0}\end{array}\right] \left[\begin{array}{r}{-1} \\ {-2}\end{array}\right] = \left[\begin{array}{r}? \\ ? \\ ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ B$ , with the corresponding elements in column $j$ of the second matrix, $ w$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ B$ with the first element in ${\text{column }1}$ of $ w$ , then multiply the second element in ${\text{row }1}$ of $ B$ with the second element in ${\text{column }1}$ of $ w$ , and so on. Add the products together. $ \left[\begin{array}{r}{1}\cdot{-1}+{3}\cdot{-2} \\ ? \\ ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ B$ with the corresponding elements in ${\text{column }1}$ of $ w$ and add the products together. $ \left[\begin{array}{r}{1}\cdot{-1}+{3}\cdot{-2} \\ {0}\cdot{-1}+{-2}\cdot{-2} \\ ?\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{r}{1}\cdot{-1}+{3}\cdot{-2} \\ {0}\cdot{-1}+{-2}\cdot{-2} \\ \color{gray}{5}\cdot{-1}+\color{gray}{0}\cdot{-2}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{r}-7 \\ 4 \\ -5\end{array}\right] $